Natural Huperzine A is a chiral molecule that is also referred to as L-Huperzine A or (−)-Huperzine A. Synthetic Huperzine A may be formed as a racemic mixture referred to as (±)-Huperzine A. Huperzine A derivatives are being developed for pharmaceutical applications.
Huperzine A (HupA), which may be extracted from a club moss (e.g., Huperzia serrata), is a sesquiterpene alkaloid and a powerful and reversible inhibitor of acetylcholinesterase (AChE). It is believed that Huperzine A has been used in China for a century or more for the treatment of swelling, fever, and blood disorders. Huperzine A has demonstrated both memory enhancement and neuroprotective effects in animal testing and human clinical trials. Recently Huperzine A has undergone double-blind, placebo-controlled clinical trials in patients with Alzheimer's disease (AD). Significant improvements have been observed in both cognitive function and the quality of life of the participants of the clinical trials. Many of the clinical trials have been conducted in China, but HupA and its derivatives are attracting considerable interest in other countries, such as those in the Western Hemisphere, where AD is a major and growing concern.
The IUPAC name of Huperzine A is (1R,9S,13E)-1-Amino-13-ethylidene-11-methyl-6-azatricyclo[7.3.1.0]trideca-2(7),3,10-trien-5-one and the chemical structure may be illustrated as follows:
Molecular Formula of Huperzine a
Huperzine A may be isolated from many kinds of plants, such as Huperziaceae, Lycopodiaceae, and Selaginella. However, the content of Huperzine A in these plants is very low. For example, the highest content of Huperzine A among the Huperzia is about 0.05 wt % based on the total weight of the plant. An investigation conducted in China between 1995 and 2001 demonstrated that the content of Huperzine A in Huperziaceae varies with the harvest time and the region in which the Huperziaceae is grown. The content of Huperzine A in Huperziaceae is in a range of about 46 μg/g to about 133 μg/g. For the eleven kinds of plants that belong to Huperziaceae Phlegmariurus, which, in general, have a higher Huperzine A content than that found in other plants, the content of Huperzine A is in a range of 242 μg/g to 560 μg/g.
In addition, the growth cycle of Huperzia serrata is about eight to ten years. The source of natural plants including Huperzine A is limited, and the extraction rate is low. Further, cutting down trees to obtain Huperzine A will cause damage to plant cover (e.g., deforestation). With the increasing number of patients with Alzheimer's disease and the shortage of natural resources, obtaining Huperzine A from plants is no longer meeting the market demand for Huperzine A. Increasingly, synthetic Huperzine A has attracted the attention of medicinal chemists.
Synthetic Huperzine A may be obtained via many different synthetic routes. Pharmacology studies indicated that the inhibitory activity of (−)-Huperzine A, with respect to acetylcholinesterase, is more than 38-50 times that of its enantiomer (+)-Huperzine A. Hence, asymmetric synthesis and chiral resolution of (−)-Huperzine A has captured attention.
The chemical synthetic methods of obtaining Huperzine A can be divided into two main types: asymmetric synthesis and chiral resolution of racemic compounds.
The asymmetric synthesis of Huperzine A is divided into two main types: asymmetric Michael-aldol reactions and asymmetric cyclization reactions catalyzed by chiral catalysts. In an asymmetric synthesis route, chiral esters may be obtained by β-ketonic ester reacted with (−)-8-phenylmenthol via transesterification. The chiral ester may then be reacted with methacrolein-DNPH via an asymmetric Michael-aldol reaction and an aldol condensation reaction, to form an intermediate. The intermediate may be transformed to methanesulfonate, and then eliminated to a diastereoisomer which may finally be purified via chromatographic separation. The asymmetric synthesis route may be illustrated as follows:

However, the above-described synthetic process is long and the yield is low. Furthermore, chromatographic separation is required and a large quantity of (−)-8-phenylmenthol is needed, and therefore, this route is not suitable for industrial production.
β-ketonic ester (β-keto ester) may also be reacted with methacrolein-DNPH via the asymmetric Michael-aldol reaction and aldol condensation reaction in the presence of a cinchona alkaloid catalyst. The reaction may be illustrated as follows:

The percent yield of the above-described reaction is only about 45% (percent yield=moles of compound 2÷moles of compound 1×100), the selectivity is low, and more than three diastereoisomers at a ratio of 10:7:1 are formed in the reaction. Therefore, dynamic axial compression chromatography is required in the post-processing.
β-ketonic ester may also react with allylpalladiumchloride dimer via an asymmetric cyclization reaction catalyzed by ferrocene. The reaction may be illustrated as follows:

Although the selectivity of the above-described reaction is relatively higher, the ferrocene is difficult to synthesize, separate, purify and recycle. Moreover, the chiral catalyst is expensive. Accordingly, this method remains limited to laboratory applications.
The chiral resolution of Huperzine A is divided into two main types: resolution by re-crystallization and resolution by chromatography. For example, two diastereomeric salts that are generated by (−)-dibenzoyl-L-tartaric acid may be reacted with a precursor of Huperzine A. One of the two salts may be re-crystallized in an organic reagent, and Huperzine A may be obtained after dissociation. The process may be illustrated as follows:

Huperzine A having high optical purity can be obtained through repeated resolutions and purification, but the process is cumbersome and the yield is very low (e.g., a percent yield of approximately 16.2%; percent yield=mass of (−)-Huperzine A in final product÷mass of (±) Huperzine A in the racemic mixture×100).